There are numerous processes utilizing gases where, due to the relatively high cost of the gas, it would be desirable to recover them. Many of such processes, however, will produce varying amounts of the gas for recovery. So, an ideal recovery system will efficiently and economically recover the gas even though the amount of gas able to be recovered varies over time. Two of these processes includes optical fiber cooling towers for the production of optical fibers and also heat treating of parts in vacuum furnaces. Those skilled in the art of gas separation will recognize that there are numerous other processes which produce such variable flows and for which recovery of a relatively expensive gas may be desirable.
In the production of optical fibers, molten glass is extruded through a die. The molten glass is rapidly quenched using a long cooling tower (draw tower). To enhance heat transfer in the cooling tower, Helium is used to as a heat transfer medium. Because Helium supplies are short and prices are increasing, capture and recycle of the Helium is desired.
The recycling of Helium from the cooling draw tower for optical fiber spinning is a demanding application. Due to addition of air to the Helium during the recovery process from the tower, extracted Helium can contain as low as 60% Helium by volume with a balance of air. It would be desirable to have a high recovery of high purity Helium. A high purity Helium product (for example, >99% vol/vol) for recycle to the cooling tower is required for cooling efficiency, while a usefully high Helium recovery is required for economic justification of the recovery process.
Typical fiber optic spinning facilities contain multiple towers. The Helium flow per tower will vary depending on the cooling needs of the tower. Conceivably, each tower can have a different Helium feed flow. For economic reasons, it would be preferable to treat multiple towers with a singe Helium recovery system. Such a potential system ideally would be able to compensate for these changes in flow. Thus, such a potential system must be able to operate with wide variation in feed flow as individual draw towers are added to service or removed from service.
One type of gas separation technology is gas separation by membranes, in particular, polymeric membranes. Membrane-based gas separation is performed by feeding a feed gas to an inlet of a gas separation membrane. Depending upon the composition of the polymeric membrane, some gases (called fast gases) will permeate across the membrane to a greater degree than other gases (called slow gases). The fast gas(es) is collected in a permeate stream while the slow gas(es) is collected a retentate or residue stream. Several have proposed the use of membranes to recover Helium from optical fiber cooling towers. In the case of glassy polymeric membranes, Helium is the fast gas while the air gases Oxygen and Nitrogen are the slow gases. Membrane systems are typically designed based on a fixed feed flow rate. In other words, the number of membrane modules of a given type of membrane is designed based upon an expected fixed flow rate of feed gas to process The number of membrane modules required for a given application is directly proportional to the feed flow. For high feed flow membrane systems, a large number of membrane modules are required. Turndown is the parameter which describes the capability of a process or system to handle changes in the feed flow relative to the maximum flow. It may be expressed in terms of the following equation:
  Turndown  =            (              1        ·                              actual            ⁢                                                  ⁢            feed            ⁢                                                  ⁢            flow                                maximum            ⁢                                                  ⁢            feed            ⁢                                                  ⁢            flow                              )        ×    100    ⁢    %  
Changes in the turndown for relatively large systems can be easily accommodated by activating or deactivating one or more of the multiple membrane modules. In short, the total membrane surface area subjected to the feed gas is adjusted to compensate for changes in feed flow.
For relatively low feed flow systems, such as optical fiber draw columns, this multiple-module approach is challenging. This is because at the maximum flow the desired product purity and recovery may be achieved with only a single commercial scale membrane module. For example, a single 1″ or 2″ diameter membrane (often the smallest commercially available membrane device) may be sufficient for the maximum flow. While the use of a single membrane module may be cost effective in terms of capital expense, unacceptable performance may be realized at flows significantly lower than the maximum flow. One potential solution to address the problem associated with such low flows is to utilize the above-mentioned multiple module approach. In order to adapt the multiple module approach to such low flows, numerous custom manufactured small permeators would need to be used. Thus, this becomes a highly customized and inefficient (cost-wise) solution.
In the heat treating of parts in vacuum furnaces, the relatively high temperature of the parts is quickly quenched with the use of inert cooling gas, such as Helium. Depending upon the amount of parts needing heat treatment, one or more of the vacuum furnaces may be placed in operation or taken out of operation. While some have proposed various strategies for recycling the cooling gas including a purification step which may involve the use of gas separation membranes. Similar to the recycling of Helium from optical fiber cooling towers, it would be preferable for economic reasons to recycle inert gases such as Helium from multiple vacuum furnaces using a single gas recovery system, such as one utilizing gas separation membranes. Such a potential system ideally would be able to compensate for a wide variation in feed flow as individual vacuum furnaces are added to service or removed from service.
With regard to Helium in particular, several have proposed various recovery strategies in the patent literature.
U.S. Pat. No. 6,517,791 describes a Helium recovery system for cold spray forming. The membrane operates in a single pass. Purification goals for the system are to increase Helium content from approximately 90% He to 97% He, a relatively narrow upgrade. In contrast, Helium recovery for optical fiber spinning often requires relatively greater enrichment of the gas.
U.S. Pat. No. 4,448,582 uses a cryogenic method for recovering Helium for recycling in an optical fiber draw tower.
U.S. Pat. Nos. 5,377,491 and 5,452,583 also pertain to recycling of Helium from an optical fiber draw tower. A membrane is described as one of several methods to purify Helium for recycle in the draw tower.
Similarly U.S. Pat. Nos. 6,092,391 and 6,253,575 B1 describe more complete Helium recovery systems for the entire optical fiber spinning process including consolidation, draw furnace and draw fiber cooling. A membrane system is described as one means for recovering the Helium.
U.S. Pat. No. 5,158,625 discloses a process for heat treating articles by hardening them in a recirculating gas medium which is in contact with the treated articles, the hardening gas being cooled by means of a heat exchanger, of the type in which Helium is used as hardening gas. At the end of a hardening operation, a Helium load is extracted from the treatment enclosure, in final phase by means of pump until a primary vacuum is obtained. The extracted Helium is brought to purifying pressure by means of a compressor associated to a mechanical filter and the Helium under purifying pressure is sent to a purifier in which impurities are removed.
U.S. Pat. No. 6,517,791 discloses a three-stage process for recovering and purifying a helium gas, and a system for using the three-stage process. A gas from a cold spray forming chamber is introduced to a particulate removing apparatus to form a particulate-free Helium gas. A first portion of the particulate-free Helium gas is recycled back to the chamber. A second portion of the particulate-free Helium gas is passed to a first compressor prior to passing a Helium gas purification membrane to form a purified Helium gas and an exhaust gas. The purified Helium gas is then passed to mix with the first portion of particulate-free Helium gas to the chamber. A third portion of the particulate-free Helium gas is passed to a liquid separator apparatus to remove water and a receiver to dampen any pulsation to form a liquid-free helium gas. The liquid-free Helium gas is recycled to the cold spray forming chamber.
Although the above patent literature discloses various solutions, none disclose methods satisfactorily addressing the issue of a broad range of feed flow rate.
Thus, it is an object to provide an improved method and system for membrane-based recovery of a gas which is adapted to achieve a sufficiently high purity over a wide range of feed flow rates.
It is another object to provide an improved method and system for membrane-based recovery of a gas which is adapted to achieve a sufficiently high recovery over a wide range of feed flow rates.
It is yet another object to provide an improved method and system for membrane-based recovery of a gas which is adapted to satisfactorily perform over a wide range of feed flow rates while incurring satisfactorily low capital costs.